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PROP chunks may appear in LISTs (not in FORMs or CATs).  They supply
shared properties for the FORMs in that LIST.  This ability to elevate
some property settings to shared status for a list of forms is useful for
both indirection and compaction.  E.g., a list of images with the same
size and colors can share one "size" property and one "color map"
property.  Individual FORMs can override the shared settings.

The contents of a PROP is like a FORM with no data chunks:

	PROP	::= "PROP" #{ FormType Property* }

It means, "Here are the shared properties for FORM type <FormType>".

A LIST may have at most one PROP of a FORM type, and all the PROPs must
appear before any of the FORMs or nested LISTs and CAT s.  You can have
subsequences of FORMs sharing properties by making each subsequence a LIST.

Scoping: Think of property settings as variable bindings in nested blocks
of a programming language.  In C this would look like:

    #define Roman           0
    #define Helvetica       1

    void main()
            int font=Roman; /* The global default */
                    printf("The font number is %d\n",font);
                    int font=Helvetica;     /* local setting */
                    printf("The font number is %d\n",font);
                    printf("The font number is %d\n",font);

    * Sample output:       The font number is 0
    *                      The font number is 1
    *                      The font number is 0

    An IFF file could contain:

    LIST {
            PROP TEXT {
                    FONT {TimesRoman}       /* shared setting       */

            FORM TEXT {
                    FONT {Helvetica}        /* local setting        */
                    CHRS {Hello }           /* uses font Helvetica  */

            FORM TEXT {
                    CHRS {there.}           /* uses font TimesRoman */

The shared property assignments selectively override the reader's global
defaults, but only for FORMs within the group.  A FORM's own property
assignments selectively override the global and group-supplied values.  So
when reading an IFF file, keep property settings on a stack.  They are
designed to be small enough to hold in main memory.

Shared properties are semantically equivalent to copying those properties
into each of the nested FORMs right after their FORM type IDs.

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